In a mobile communication system of the LTE (Long Term Evolution) scheme defined in the 3GPP, a radio base station eNB is configured to perform the MIMO (Multi Input Multi Output) multiplexing transmission in which multiple different streams (data sequences) are simultaneously transmitted using multiple antennas to a mobile station UE.
The MIMO multiplexing transmission is characterized in that once a correlation of a radio propagation path between each transmitting antenna and a mobile station UE (fading correlation) increases, a reception quality (reception error rate) deteriorates.
Further, the MIMO multiplexing transmission is characterized in that when the number of streams to be simultaneously transmitted to the mobile station UE is decreased, a transmission rate decreases in a downlink but the reception quality improves.
Therefore, in the mobile communication system of the LTE scheme, in consideration of the characteristics of the above-described MIMO multiplexing transmission, the radio base station eNB is configured to adopt “Rank Adaptation control” in which the number of streams to be simultaneously transmitted to the mobile station UE is controlled based on “RI (Rank Indicator)” indicating the “Rank” of a radio propagation path estimated by the mobile station UE.
In this case, the “Rank” is calculated by the mobile station UE based on a fading correlation and a measurement value of reception SIR (Signal-to-Interference Ratio), and denotes the number of streams by which it is estimated to achieve the maximum throughput.